Observations of heterogeneous pore pressure distributions in clay-rich materials

Robert Cuss, Jon Harrington, Caroline Graham, Shanvas Sathar, and Tony

Milodowski

British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

0115 936 3486

rjcu@bgs.ac.uk

Abstract

The concept of effective stress is one of the basic tenets of rock mechanics where the

stress acting on a rock can be viewed as the total stress minus the pore water

pressure. In many materials, including clay-rich rocks, this relationship has been seen

to be imperfect and a coefficient (χ) is added to account for the mechanical properties

of the clay matrix. Recent experimental results seen during the flow testing (both gas

and water) of several rocks (Callovo-Oxfordian claystone, Opalinus Clay, Boom

Clay) and geomaterials (bentonite, kaolinite) has given evidence for stable high

pressure differentials. The design of experimental setups gives multiple measurements

of pore pressure, often showing a complex distribution for several different

experimental geometries. The observed stable high pressure differentials and

heterogeneous pore pressure distribution makes the describing of stress states in

terms of effective stress complex. Highly localised pore pressures can be sustained by

argillaceous materials and concepts of evenly distributed pore pressures throughout

the sample, i.e. conventional effective stress, do not fit many clay-rich rocks if the

complexities observed on the micro-scale are not incorporated, especially when

considering the case of gaseous flow.

Introduction

The introduction of pore-fluid under pressure has a profound effect on the physical

properties of porous solids (Hubbert and Rubey, 1961; Terzaghi, 1943). In a saturated

porous system, the fluid supports some proportion of the applied load, creating fluid-

pressure, which acts in the opposite direction to load, lowering the overall stress

exerted through grains. The addition of fluid pressure lowers available stress by an

amount that is proportional to the pore pressure. Therefore the law of effective stress

states:

σ' = σ – u

where σ' is effective stress, σ is total stress, and u is pore pressure. Strength is

determined not by confining pressure alone, but by the difference between confining

and pore-pressures. In simple drained tests, pore pressure remains constant at

atmospheric pressure and the observed effective stress is similar to the applied load.

Conversely, if the pore-fluid system is closed, pore pressure rises in proportion to the

applied load as pore space is reduced, significantly lowering the overall effective stress.

Thus, the mechanical response of rocks to applied load is significantly affected by the

ability of fluids to drain.

Many rocks have been shown to follow the law of effective stress, including shale

(Handin et al., 1963; Kwon et al., 2001) and sandstone (Byerlee, 1975; Cuss, 1999).

However, due to the compressibility of clay and the poroelastic effect (after Biot,

1941), where the effective stress is modified by the partial transfer of pore-pressure to

the granular framework, the law of effective stress is modified with the effective

pressure coefficient (χ):

σ' = σ – χ u

Kwon et al. (2001) showed that the effective pressure coefficient χ was equal to 0.99

± 0.06 for Wilcox shale. This value is indistinguishable from unity and demonstrates

that the law of effective stress is obeyed in this particular shale variety. Burrus (1998)

showed that χ much less than unity was appropriate for shale for the Mahakam Delta

(χ = 0.65 – 0.85).

Conceptual models of argillaceous rock deformation, such as critical state soil

mechanics (Schofield and Wroth, 1968; Atkinson and Bransby, 1978; Wood, 1990),

use the law of effective stress. Therefore careful experimentation has to be undertaken

to account for drainage effects and to determine the role that pore pressure has on the

deformation of sediments. This paper gives examples of observations of pore pressure

in a number of experimental studies, which make the defining of effective stress

difficult in low permeability argillaceous materials.

In recent years the British Geological Survey (BGS) has undertaken a number of fluid

transport experimental studies using isotropic, triaxial, shear and full-scale

demonstration testing configurations. This paper draws together a number of

observations of pore pressure heterogeneity for various argillaceous materials. These

localised heterogeneities give rise to additional complication of the law of effective

stress and may be resultant of the mechanisms of fluid flow. Table 1 summarises the

experimental boundary conditions of the different test geometries and the basic

physical properties of the test materials.

Experimental geometry 1 – Triaxial testing

The Stress-Path Permeameter (SPP, Figure 1) was used to investigate water and gas

(helium) flow in Callovo-Oxfordian claystone from the Bure Underground Research

Laboratory (URL) in the eastern part of the Paris Basin under in situ conditions (see

Table 1 for test material parameters and experimental boundary conditions). The

triaxial SPP testing rig was designed to observe sample volume changes during flow

experiments conducted along an evolving stress-path; please note that the results

presented here were for a static triaxial boundary condition. The apparatus had a

thick-walled pressure vessel which imposed a confining pressure (12.5 MPa) through

the compression of glycerol by an ISCO syringe pump. The testing sample was

cylindrical and of 56 mm diameter and 82 mm length. The sample was jacketed in a

Hoek-sleeve, which had been thinned to reduce compressibility and had three brass-

plates cemented to the outside of the jacket. Three pressure-balanced devices (dash-

pots) allowed direct measurement of the displacement of these plates to give radial

displacement. The sample was axially loaded by an Enerpac hydraulic ram driven by

an ISCO syringe pump, giving a stress of 13 MPa in the axial direction. Axial

displacement was directly recorded by a Mitutoyo digital micrometer. The pore-

pressure system had two ISCO syringe pumps; one acted as an injection pump and the

other maintained constant back-pressure (4.5 MPa). A test was conducted using stages

of constant pressure or constant-flow pressure ramps in the injection system in order

to initiate gas or water flow. The experimental rig was completed by a number of

pressure sensors, thermocouples and a digital acquisition system, which logged data

every 2 minutes.

One experimental uncertainty in transport testing is the short-circuiting of the flow

system along the jacket of the test sample. The addition of a 6 mm wide, 2mm deep,

porous stainless-steel annular filter along the outer edge of each platen (Figure 1b)

allowed pore-water pressure to be monitored and discount unwanted sidewall flow.

The inlet/outlet filter was made up of a porous disc 20 mm in diameter and 2 mm

depth. The two guard-rings were each connected to a pressure transducer and the

complete guard-ring system (filter, pipework and pressure sensor) was saturated with

water and flushed in order to eliminate gas from the system. The control board of the

apparatus allowed the guard rings to be either connected to the injection system to

assist in hydration, or isolated to give an independent measure of pore-pressure. As

well as being able to eliminate side-wall flow as a possible transport mechanism, the

guard rings meant pore pressure was measured at four different points on the test

sample (injection pressure, injection guard-ring pressure, back pressure, back guard-

ring pressure) providing data on the hydraulic anisotropy and symmetry within the

sample.

The SPP was used to conduct experiments on the Callovo-Oxfordian claystone (COx)

under in situ stress conditions. Wenk et al. (2008) reports clay 25-55 wt%, 23-44%

carbonates and 20-31% silt (essentially quartz + feldspar). Clay minerals are reported

to include illite and illite-smectite with subordinate kaolinite and chlorite. The test

sample was prepared by dry machine lathing and the ends were ground flat and

parallel giving an 82.45 mm length sample. The starting water saturation of the

sample was 96 %, with the early stages of testing designed to raise this to full

saturation. Table 1 summarises the geotechnical properties of the starting material, on

completion of the full test programme the sample was seen to have saturation close to

100 %.

Observed pore pressure distribution under in situ stress conditions

The first stage of testing imposed stress conditions similar to the in situ conditions for

the borehole the test sample was taken from (σ1 = 13 MPa, σ2 = σ3 =12.5 MPa, Pp =

4.5 MPa). Chemically balanced water was injected at both ends of the sample in order

to re-establish full sample saturation and to minimise chemically driven swelling of

the sample. During extraction, storage and sample preparation the sample will have

undergone small amounts of desaturation and this stage of the test counteracts this

effect. During this initial resaturation stage, the inlet/outlet and guard rings were used

to hydrate the sample and ensured that the air content of all pipework and pressure

transducers was negligible. Following the initial resaturation stage, a two-step

constant head test was conducted, as shown in Figure 2a. At the start of this test stage

the injection pore pressure, including the guard ring, was raised to 8.5 MPa and then

the guard-rings were isolated. Both injection (IGR) and back-pressure (BGR) guard

rings showed an initial decrease in pore-pressure, which within a couple of days

started to recover in pressure and equilibrated. The IGR stabilised approximately 1

MPa below the injection pressure, whereas BGR was about 0.5 MPa above the back

pressure.

Injection pore pressure was lowered to 4.5 MPa on Day 98. Over the subsequent

remaining 32 days of the low pressure constant head hydraulic test stage a slow

decrease in pore pressure in IGR and BGR was seen, with BGR decreasing much

slower than IGR.

The data show that a relatively even distribution in pore pressure is created in the

sample due to the high pressure imposed at the injection end of the sample. This

suggests a relatively evenly distributed pore pressure gradient through the sample.

The difference in pore pressure over a relatively small distance suggests a high

pressure gradient of approximately 100 MPa/m, which resulted in a flow of only

2.8 × 10-13 m3.s-1.

Following the hydraulic head test, a helium gas injection test was conducted. As

shown in Figure 2b, the response of the guard rings is much more complicated during

gas testing. The onset of gas flow was higher than anticipated and hence the need for

the slow careful raising of gas pressure at the injection inlet. Gas pressure was raised

by means of a constant flow pressure-ramp to raise pressure to 9.5 MPa over 30 days.

Pressure was then held constant, before a series of pressure ramps slowly increased

gas pressure by 0.5 MPa each step.

Figure 2b shows that BGR pressure showed little pressure change during the first

pressure ramp, whilst IGR pressure showed a small increase. Prior to the switch to

constant pressure the guard ring pressure data showed an increase; IGR pressure

peaked at ~5.2 MPa and slowly decayed. This corresponded with no change of inflow

into, or outflow from, the sample. Up until approximately Day 237, the pressure

within the guard rings appeared to be independent of the injection gas pressure. This

showed that within the test sample very little pore pressure change had occurred and

gas had not started to enter the sample. Even accounting for the differences in

compressibility of helium and water, the “bulk” sample pore pressure was relatively

unaffected by the injection pressure.

The final pressure ramp was initiated on Day 236 and raised pore pressure by 0.5 MPa

over a 3 week period; pressure in IGR started to increase from Day 236 onwards. It is

difficult to determine precisely when BGR pressure started to increase, however there

was a lag of approximately one day compared with IGR. Between Day 237 and Day

254.7 the pressure in IGR increased from 4.6 MPa to 6.3 MPa, whilst in BGR

pressure increased from 4.5 MPa to 4.9 MPa. At Day 254.7 pressure in IGR increased

significantly and rapidly from 6.3 MPa to a peak of 10.5 MPa.

The observed pore pressure response showed that little change occured prior to the

onset of gas flow into the sample. Once this occurs there is a considerable difference

in pore pressure within the sample. Pore pressure is not as evenly distributed

throughout the sample as was seen during the hydraulic head test. The maximum

pressure difference observed equates to a gradient of 400 MPa/m.

The differences between pore pressure response for water and gas flow suggest that

the flow mechanisms are different. Harrington et al. (2012) show that at the onset of

gas flow Callovo-Oxfordian claystone undergoes volumetric dilation. This suggests

that gas flow was along localised dilatant pathways (micro-fissuring) which may or

may not interact with the continuum stress field; also termed dilatancy flow. As well

as strain data, dilatancy flow in argillaceous materials has also been suggested from

studies of nano-particle invasion in Boom Clay (Harrington et al., 2012) and during

the heating of Opalinus Clay in glycerol (Harrington et al., 2003), which suggested

that gas flow in this material was through a small number of discrete pathways. All of

these observations suggest that water flow at elevated pore pressures results in

pervasive changes in pore pressure, whereas gas flow at elevated pressures has a

much more localised influence. This suggests that an effective stress law may usefully

be applied to the case of water as a pore fluid, but may only be applicable on a local

scale for gas flow.

Experimental geometry 2 – interface testing

The Angled Shear Rig (ASR, Figure 3a) was used to investigate water and gas flow

within a kaolinite gouge sandwiched between two steel platens angled to the stress-

field (see Table 1 for test material parameters and experimental boundary conditions).

Kaolinite pastes were prepared and placed between two polished steel blocks, which

maintained a 60 mm × 60 mm contact area. The apparatus was designed with the

following specification; normal load of up to 12 MPa, injection pore pressure of 0.5 –

12 MPa, vertical displacement measured to a precision greater than 60 nm using an

induction device, “fracture” thickness measured to sub-micron accuracy by an eddy

current device, and pore pressure measured at two locations on the slip-plane (see

Figure 3a for locations). See Sathar et al. (2012) for a more detailed experimental

description. The results presented here were not recorded during an active shear

experiment, however small shear movements (<5 microns) occur early during the

loading history creating a shear stress in the gouge material.

Observed pore pressure distribution during interface testing

Figure 4 shows the results from two experiments conducted using the ASR apparatus.

Figure 4a shows the results from a water injection test (ASR_Tau01), with a constant

injection pressure of 0.75 MPa, the normal load was increased and decreased in steps

from 0.2 MPa to 3 MPa to 0.1 MPa. As injection pore pressure was raised to 750 kPa,

flow initiated and the pore pressure within the slip-plane increased with a time-

dependent response until reaching a peak of about 24 kPa. As normal load was

stepped up and down from 2 MPa to 3 MPa to 2 MPa the pore pressure in the slip-

plane remained relatively unchanged, apart from the initial time-dependent response.

One of the slip-plane pore pressure sensors shows a slow decay, whereas the other

maintains a constant pressure. After 20 days the pore pressure was allowed to leak-

off, the rate of pressure decay was slow due to the low permeability of the compressed

kaolinite paste. Leak-off can be seen to have a marked influence on the pore pressure

within the slip-plane. Therefore injection pressure was the primary control on pore

pressure distribution during water injection testing. It should be noted that the pore

pressure in the slip-plane was of the order of 20 kPa with an injection pressure of 750

kPa. This suggests that pore pressure drop-off from the central injection filter was

greater than simple radial flow would predict or that flow was localised within the

kaolinite. The response of the pore pressure on the slip-plane appears to mainly be

due to drainage along the slip- plane and the injection pressure and is not a function of

normal load.

Figure 4b shows the result of pore pressure on the slip-plane during gas testing (test

ASR_Tau12). In order to initiate flow the gas pressure was slowly increased from 3.5

MPa up to 5 MPa. Once flow had been initiated the normal load was stepped up and

down from 0.3 MPa to 3 MPa to 0.3 MPa. During the full duration of this test (and

subsequent gas tests) no response was observed in the pore pressure within the slip-

plane. This suggests that gas flow was localised and that its impact on pore pressure

within the slip-plane was minimal.

Experimental geometry 3 – shear testing

The Direct Shear Rig (DSR, Figure 3b) was used to investigate fracture transmissivity

of Opalinus Clay from the Mont Terri Underground Research Laboratory in the

northwest of Switzerland under realistic in situ loading conditions (see Table 1 for test

material parameters and experimental boundary conditions). The DSR is similar to the

Angled Shear Rig, but is designed to test rock samples. The apparatus was designed

with the following specification to test two prepared samples of 60 mm × 60 mm

× 21 mm; normal load of up to 12 MPa, injection pore pressure of 0.5 – 12 MPa,

shear as slow as 1 mm per 3 month period, and vertical displacement measured to a

precision greater than 60 nm. Pore pressure can be injected directly to the fracture

plane by means of a 4 mm diameter injection pipe (See Cuss et al., 2011b for a more

detailed description).

Opalinus Clay has the following composition; clay-mineral content ranges from 40 –

80 wt % (9 – 29 % illite, 3 – 10 % chlorite, 6 – 20 % kaolinite, and 4 – 12 %

illite/smectite mixed layers in the ratio 70/30). Other minerals are quartz (15 – 30 %),

calcite (6 – 40 %), siderite (2 – 3 %), ankerite (0 – 3 %), feldspars (1 – 7 %), pyrite

(1 – 3 %), and organic carbon (<1 %). The total water content ranges from 4 – 19 wt

% (Gautschi, 2001). Table 1 summarises the geotechnical properties of the starting

material.

Observed pore water distribution during shear testing

Figure 5 shows the results from water injection test DSR_OPA-1 conducted at 1.2

MPa normal load with an injection pressure of 1 MPa at a shear rate of 85

microns/day. Cuss et al. (2011b) describes the full test history in detail. Late in the test

history after nearly 6 mm of shear had occurred, fluorescein was added to the

injection fluid to allow imaging of the distribution of flow along the fracture plane. As

shown in Figure 5, even during water transport along a confined planar interface, flow

was highly variable and far from perfectly radial. Flow preferentially followed micro-

scale fractures that formed in the fracture surface. Water flow paths are seen to be

complex and only about a quarter of the fracture plane contributed to overall flow.

Experimental geometry 4 – Full scale demonstration (Lasgit)

The on-going Large scale gas injection test (Lasgit) is a full-scale experiment based

on the Swedish KBS-3V repository concept, examining the processes controlling gas

and water flow in compact buffer bentonite (see Cuss et al., 2010). The experiment

was commissioned in a 9 m deep, 1.75 m diameter deposition hole at 420 m depth

within the Äspö Hard Rock Laboratory (HRL) in Sweden. The experiment aimed to

perform and interpret a series of gas injection tests in a full-scale mock-up of the

KBS-3V disposal concept. Issues relating to up-scaling were examined and its effect

on gas movement and buffer performance. The test data allowed processes governing

gas migration to be investigated and allowed testing/validation of modelling

approaches aimed at repository performance assessment.

A full-scale KBS-3 canister (see Figure 6 for schematic) was modified with thirteen

circular filters of varying dimensions located on its surface to simulate potential

canister defects. These filters could also be used to inject water during hydration

stages to help locally saturate the buffer around each test filter. The canister was

surrounded by specially manufactured pre-compacted bentonite blocks, all of which

had initial water saturations in excess of 95% (see Table 1 for test material parameters

and experimental boundary conditions). The deposition hole, buffer and canister were

equipped with instrumentation to measure the total stress, pore-water pressure and

relative humidity in 32, 26 and 7 positions respectively (see Figure 6 for the location

of pore-water sensors). Additional instrumentation continually monitored variations in

temperature, relative displacement of the lid & canister, and the restraining forces on

the rock anchors. The emplacement hole had been capped by a conical concrete plug

retained by a reinforced carbon steel lid. The experiment was monitored and

controlled from a temperature controlled “Gas Laboratory" that allowed remote

control and monitoring of the test to be undertaken by project staff remotely.

Observed pore pressure distribution in full-scale testing

The size of Lasgit has allowed pore pressure to be monitored at 50 locations for in

excess of 6.5 years. Figure 7 shows the results for a number of pore pressure sensors

during the full history of the experiment. Following initial hydration pore pressure at

the rock wall (UR sensors, Figure 7a, Figure 6) increased to between 1,250 and

2,700 kPa and since reaching a maximum about Day 600, UR sensors have generally

decreased, on average at a rate of approximately 40 kPa per year. This is thought to be

due to the change in drawdown of the main tunnel of the Äspö HRL.

Pore-water pressure within the bentonite blocks (UB sensors, Figure 7b, Figure 6) had

a much more complex history. Generally pore pressure remains low at 220 – 750 kPa,

except for UB902, which was pressured during the second gas injection test when gas

reached this location. The pore pressure history was complex with periods of pressure

increase and decrease, with some evidence of seasonal variation. However, the

seasonal variation was non-uniform and non-repeatable in some sensors.

While the test had been continually hydrated at 2.5 MPa, pore pressure distribution

within the deposition hole remains complex ranging from 220 to 2,500 kPa. While the

bentonite continues to mature, hydraulic constant head tests at various times in the test

history and psychrometer data from seven locations within the bentonite show that the

buffer is in hydraulic disequilibrium. Even taking this into account the distribution of

hydraulic pore pressure is complex on the full-scale.

Figure 8 shows the observed pore pressure during the second gas injection test (see

Cuss et al., 2011a for more detail). Gas pressure was increased in one canister filter

(FL903, section 5 Figure 6) from 2.5 to approximately 6 MPa. During this

pressurisation no other filter showed a change in pore pressure. At gas breakthrough

at Day 1769.7 when gas flow was initiated, none of the lower canister filters showed a

change in pore pressure, whereas only 6 pore pressure sensors on the rock wall (UR

sensor) showed any variation. The maximum disturbance seen in the UR sensors was

an increase of ~50 kPa and this was likely to be a hydro-mechanical response.

Examination of pore pressure sensor data indicates that gas migrated temporally

within the system to different locations. As seen in Figure 8a, pore pressure in filter

FL901 started to increase in a series of steps initiating at Day 1772.9, some 3.2 days

after gas breakthrough. Pore pressure greatly increased in sensor UB902 (Figure 8b)

within the bentonite at Day 1785.4, some 15.7 days after gas breakthrough. For both

of these events no significant change in pore pressure is observed elsewhere in the

system suggesting that gas migrated directly to these sinks. These observations show

that gas flow was localised within the Lasgit deposition hole and that pore pressure

was not evenly distributed throughout the deposition hole.

Discussion and implications

The mechanical deformation of argillaceous rocks is usually described in terms of

effective stress, which is one of the basic tenets of rock mechanics. In order to do this

it is important accurately to describe pore pressure. In this paper we have shown that

pore pressure distribution during different laboratory and full-scale experiments was

far from simple during both water and gas injection testing. This complexity, even

taking into account anisotropy and material heterogeneities, is not easily described by

a single value of χ in the law of effective stress as we cannot answer the simple

question “what is the pore pressure of the bulk sample?”

Laboratory triaxial experiments suggest that hydraulic pore pressure can be described

as an evenly distributed quantity that is dictated by the boundary conditions of the

experiment. However, the pore pressure distribution observed in shear experiments

and in the full-scale Lasgit test suggest that pore pressures are not evenly distributed

and can be localised into almost isolated areas, although in the case of Lasgit this may

stem from disequilibrium. This is most obvious in the shear experimental results

where only approximately a quarter of the fracture surface is contributing to the flow

of water. Observations within Lasgit show that pore pressure is very heterogeneous,

with high pore pressures seen at the rock wall and low pressures seen within the

bentonite. Constant head testing has no observable effect on other pore pressure

sensors and suggests that pore pressure variation is localised to a relatively small zone

around the injection filter.

The localisation of pore pressure variation in Lasgit may be explained by the

mechanical compressibility of the bentonite buffer and its low permeability. The

matrix of the buffer was able to accommodate the displaced pore water by

compression, whereas on the sample scale in the laboratory the pressure differences

are seen to be explainable by even distribution of pore pressure within the pore

network. However, this cannot explain the non-radial flow seen in shear-box testing.

The observations of pore pressure variation during gas injection show that gas

transport mechanisms are dissimilar to water flow. Pore pressure observations

consistently show localised increases in pore pressure with gas flow along a number

of potentially isolated dilatant pathways (micro-fissures). These observations suggest

that under these circumstances a meaningful average pore pressure could be difficult

to define. Gas pressure within the micro-fissure pathway can be much greater than the

pore pressure in the clay surrounding the pathway. A certain amount of pore pressure

increase and pore water displacement will occur close to the pathway as a

combination of dilation, which will result in localised compression of the clay

material, increasing pore pressure through compression of the micro-fissure wall, and

the transmission of gas pressure directly through the pathway wall due to the loss of

capillary pressure. The driving force of micro-fissure propagation is the gas pressure

at the tip of the dilatant feature, which is much greater than the “bulk” pore pressure

of the sample. As seen in Lasgit, the majority of the system sees no change in pore

pressure as gas pressure increases in an injection filter and even when gas becomes

mobile and enters the buffer the pore pressure in the greater part of the system does

not vary. Individual sensor responses in triaxial and full-scale testing show that

argillaceous materials see no change in pore pressure until gas migrates near to that

location and that pressure build-up is then seen to develop quickly.

Consistent behaviour is observed in a range of argillaceous materials for different

experimental geometries; including isotropic (Boom Clay), triaxial (Callovo-

Oxfordian claystone), interface (kaolinite), shear (Opalinus Clay) and full-scale

testing (bentonite). This suggests a commonality of behaviour independent of

experimental setup and a common underlying physical mechanism involved in gas

and water flow. Only relatively minor differences are observed between argillaceous

materials, which may be explained by differences in material properties and state of

induration. In all of these argillaceous materials it is difficult to determine a bulk pore

pressure and thus the use of the law of effective stress may introduce an error into

model predictions unless the complexities observed at a micro-scale are incorporated.

The experimental observations are problematic for existing theoretical frameworks,

which generally assume homogeneous distributions of pore-pressure. Many of the

observed experimental features suggest highly localised increases in pore pressure and

a heterogeneous pore pressure distribution. These observations support the hypothesis

of dilatancy flow along micro-fissures being the dominant physical mechanism

controlling gas flow. Although not described fully in this paper, Harrington et al.

(2012) give further evidence of dilatancy flow. Therefore on a micro scale the law of

effective stress will describe the deformation of the clay matrix, but on a bulk scale

care has to be taken as the “bulk” pore pressure may vary considerably depending on

the number, geometry and distribution of gas pathways.

It is important to investigate the controls on gas migration and what controls the

formation of dilatant pathways from the micro, macro and field scales. These

observations then need to be considered in the development of theoretical frameworks

describing stress in such systems.

Conclusions

Large, stable pressure differentials and gradients were observed in several

argillaceous materials during water and gas injection testing for a number of

experimental geometries, including isotropic (Boom Clay), triaxial (Callovo-

Oxfordian claystone), shear (kaolinite and Opalinus Clay) and full-scale testing

(bentonite). Pore-pressure during water injection appeared to be evenly distributed on

the sample scale, whereas in full-scale demonstration a complex distribution was

seen, which may partly be due to hydraulic disequilibrium. During gas injection

testing all observations suggest that transport was predominantly by dilatancy flow

and the formation of micro-fissures. This led to localised pore pressure variations and

a complex temporally and spatially varying pore pressure distribution. Isolated

pockets of increased gas pressure could be seen to be stable.

The nature of pore-pressure distribution, both hydraulic and gaseous, and the stability

of pore pressure differentials means that the description of a meaningful average pore

pressure was difficult and thus the use of effective stress with a single χ value might

misrepresent local stresses within the sample. Localised deformation in the formation

of dilatant pathways was dominated by the local gas pressure and not the bulk pore

pressure. Therefore the law of effective stress on the micro-scale will be valid,

whereas on a bulk scale could lead to errors in model predictions.

Acknowledgements

This study was undertaken by staff from the British Geological Survey (Radioactive

Waste Team) and is published with the permission of the Executive Director, British

Geological Survey (NERC). Some of the research leading to these results has received

funding from the European Atomic Energy Community’s Seventh Framework

Programme (FP7/2007-2011) under Grant Agreement no230357, the FORGE project.

The authors gratefully acknowledge the co-funding of this research by Andra

(France), Nagra (Switzerland), Nuclear Decommissioning Authority – Radioactive

Waste Management Division (UK), SCK-CEN (Belgium), and SKB (Sweden).

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Triaxial geometry Interface geometry

Shear geometry

Full scale Units S

amp

le p

rop

erti

es

Sample reference SPP_COx-2 ASR_Tau01 and

ASR_Tau12 DSR_OPA-1 Lasgit

Material Callovo-Oxfordian

Claystone Kaolinite Opalinus Clay Bentonite

Location Bure URL, France ECC quarry, St

Austell, UK Mont Terri URL,

Switzerland Äspö HRL,

Sweden

Borehole / drill core

OHZ1201 / EST30341

/ BHA-8 DA3147G01

Core direction Parallel to bedding / Perpendicular to

bedding /

Sample geometry Cylindrical sample Gouge material Block sample Compacted

ring

Average length 82.45 ± 0.03 60.0 ± 0.08 20.61 ± 0.04 7,530 ± 5 mm

Average diameter / width

55.85 ± 0.04 0.06 – 0.08 59.62 ± 0.14 1,632 ± 1 mm

Volume 2.020 × 10-4 2.52 × 10-7 1.471 × 10-4 11.23 m3

Average weight 495.02 / 352.66 2.29 × 107 g

Density 2.451 / 2.397 2.056 g.cc-1

Grain density 2.7 2.62 2.69 2.77 g.cc-1

Moisture weight 28.7 / 20.45 4.43 × 106 g

Moisture content 6.2 / 5.8 24.0 %

Dry weight 466 / 332.21 1.85 × 107 g

Dry density 2.31 / 2.257 1.65 g.cc-1

Void ratio 0.174 / 0.192 0.687

Porosity 14.8 / 16.08 40.6 %

Degree of saturation

96 / 81.4 97.1 %

Gravimetric water content

/ 80 / / %

Exp

erim

en

tal

bo

un

dar

y c

on

dit

ion

s Confining

pressure 12.5 / / In situ MPa

Axial load 13 / / In situ MPa

Pore pressure 4.5 – 10.5 0.75 and 5 1 1.5 – 6 MPa

Back pressure 4.5 Atmosphere atmosphere In situ MPa

Normal load / 0.2 – 3 – 0.1 1.2 / MPa

Shear rate / None 85 / μm/day

Pore fluid chemistry

Chemically balanced pore fluid for Bure#

De-ionised water / helium

Chemically balanced pore fluid for Mont

Terri @

Water / neon

Table 1 Description of pre-test material properties and experimental boundary conditions for the

four test geometries discussed. # - 227 mg.l-1 Ca2+, 125 mg.l-1 Mg2+, 1012 mg.l-1 Na+, 35.7 mg.l-1 K+,

1266 mg.l-1 SO42-, 4.59 mg.l-1 Si, 9.83 mg.l-1 SiO2, 13.5 mg.l-1 Sr, 423 mg.l-1 total S, and 0.941 mg.l-

1 total Fe. @ - 7.598 g.l-1 NaCl, 0.231 g.l-1 KCl, 0.496 g.l-1 MgCl2, 0.803 g.l-1 CaCl2, 1.420 g.l-1

Na2SO4 and 0.033 g.l-1 Na2CO3 (Pearson et al., 1999).

a).

b). c).

Figure 1 – Triaxial test setup; a) triaxial Stress-Path Permeameter (SPP) apparatus, b) end

platen showing the injection filter and guard ring arrangement, c) sample of Callovo-Oxfordian

claystone prior to testing.

a).

b).

Figure 2 – Results of pore pressure recorded during hydraulic and gas testing of Callovo-

Oxfordian claystone a) during water-injection testing, b) during gas injection. Differences can be

clearly seen between the distribution of pore pressure within the sample during water and gas

injection.

a).

b).

Figure 3 - Schematic views of a) the Direct Shear Rig (DSR) and b) the Angled Shear Rig (ASR).

a).

b).

Figure 4 – Observations of pore pressure on the slip-plane during testing without active shear a)

during water-injection testing, b) during gas injection. Relatively low pore pressures are

observed within the slip-plane and the evolution of pore-pressure appears independent of the

normal load on the slip-plane.

Figure 5 – Distribution of fluorescein staining during shear testing of Opalinus Clay. Laser-

stimulated scanning fluorescence image (LSSFI) under blue (450 nm) excitation superimposed

onto photomicrograph of the post-experiment contact surface of the upper clay block. The

relative concentration of the fluorescein is indicated by the rainbow colour scale [RED = high

concentration; BLUE = absent].

Figure 6 – Schematic of the layout of the Lasgit experiment showing the locations of sensors displayed in Figure 7 and Figure 8.

a)

b)

Figure 7 – Hydraulic pore-pressure seen in Lasgit a) at the rock wall, b) within the bentonite

buffer. Pore pressure at the rock wall greatly increased around day 500 following the installation

of a pressure-relief hole. Since this date the pressure has decayed due to changes in drawdown of

the Äspö tunnel. Pore pressure within the bentonite is much more complex and has evolved to a

magnitude much less than observed at the rock wall.

a)

b)

Figure 8 – Pore-pressure seen in Lasgit during gas testing a) in canister filters, b) within the

bentonite buffer. Gas pressure was raised in filter FL903 until gas entry into the bentonite buffer.

Pressure increases in filter FL902 and UB902 sometime after gas breakthrough illustrate the

temporal migration of gas within Lasgit.